1. Field of the Invention
The present invention relates to a method of crystal growth of a compound semiconductor, and more specifically to a method of crystal growth of a GaInNAs-type compound semiconductor by a vapor phase growth method. The invention also relates to a compound semiconductor device having as a part of the device a Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y or GaN.sub.y As.sub.1-y crystal thin film. The invention further relates to a method of manufacturing such a compound semiconductor device.
2. Description of the Background Art
First of all, the background art relating to the compound semiconductor device using GaInNAs will be described.
As mentioned earlier, in recent years, a group III-V mixed crystal semiconductor containing nitrogen as a group V element has received much attention as a novel semiconductor material. With this material, through appropriate selection of the concentrations of nitrogen and component elements, epitaxial growth without misfit dislocation on a Si, GaAs, InP, or GaP substrate is possible.
For instance, Document 1 (Japanese Patent Laying-Open No. 6-334168) describes an example in which a group III-V mixed crystal semiconductor is epitaxially grown on a Si substrate to produce an Si electronic device or the like having a monolithic structure. In addition, Document 2 (Japanese Patent Laying-Open No. 6-37355) describes the examples in which GaInNAs, AlGaNAs, or GaNAs is epitaxially grown on a GaAs, InP, or GaP substrate. Document 3 (Japanese Patent Laying-Open No.9-283857) describes an example in which a GaInNAs thin film crystal is epitaxially grown on a GaAs substrate to form a semiconductor laser.
The following is the possible advantages of forming an optical device or an electronic device using a group III-V mixed crystal semiconductor containing nitrogen on the GaAs substrate, for example, Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y or GaN.sub.y As.sub.1-y. Conventionally, most of the mixed crystal semiconductors, the examples of which include AlGaAs, GaInP and such, that are lattice matched to the GaAs substrate have a band gap larger than that of GaAs. Here, the new materials, Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y and GaN.sub.y As.sub.1-y, have the advantage of making the band gap smaller than that of GaAs.
In addition, the band gap may be advantageously altered continuously by changing the In composition x and the nitrogen concentration y for Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y and the nitrogen concentration for GaN.sub.y As.sub.1-y. When this material is combined with other materials to form a multi-layer structure, an optical device having a light-emitting wavelength longer than that of GaAs can be produced, which has hitherto been impossible. For instance, by using Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y or GaN.sub.y As.sub.1-y for the active layer, it is possible to produce a semiconductor laser which lases at 1.3 .mu.m or 1.55 .mu.m used in optical fiber communications. A light-receiving diode for detecting the infrared light can also be produced.
FIGS. 1A and 1B show the example (a) of a semiconductor laser and the example (b) of a photo diode using Ga.sub.0.85 In.sub.0.15 N.sub.0.05 As.sub.0.95. For the semiconductor laser, the above composition allows the lasing operation at 1.3 .mu.m utilized for optical communications using optical fibers.
Ga.sub.0.85 In.sub.0.15 N.sub.0.05 As.sub.0.95 is used for an active layer. When Ga.sub.0.85 In.sub.0.15 N.sub.0.05 As.sub.0.95 is used to form the p-i-n structure consisting of stacked layers of p-type having a high carrier concentration, of n-type having a low carrier concentration, and of n-type having a high carrier concentration, the light-receiving diode can detect infrared light of up to 1.3 .mu.m.
Until now, these laser diodes and photo diodes have been formed on InP substrates. In the laser diode, InGaAsP has been used for the active layer. The InP substrate, however, is inferior to the GaAs substrate both in the aspects of mass-productivity and cost. The lasers formed on such substrates are also inferior with regard to their mass-productivity and cost.
Although GaN.sub.y As.sub.1-y does not lattice match to GaAs, the diode structure can be formed without the development of a crystal defect such as misfit dislocation by keeping the thickness sufficiently thin when GaN.sub.y As.sub.1-y is used for the active layer of a laser diode. Similarly, although misfit dislocation may occur when using Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y, if the selection of the x to y composition ratio is inappropriate, the development of a crystal defect can be prevented by keeping the thickness sufficiently thin.
Next, the background art relating to the method of crystal growth of a compound semiconductor will be described. In order to grow a crystal of a group III-V compound semiconductor by a vapor phase growth method, a group III source material supplying a group III element and a group V source material supplying a group V element are used.
When growing a crystal of a group III-V compound semiconductor such as a ternary GaNAs-type compound semiconductor, a nitrogen-containing group V source material supplying N and an arsenic-containing group V source material supplying As are used together as the group V source material. Conventionally, nitrogen (N.sub.2) gas or, as disclosed in a Document 4 (Journal of Crystal Growth 145 (1994), pp. 99-103), a nitrogen source produced by ammonia (NH.sub.3) activated by RF plasma is used as the nitrogen-containing group V source material. Moreover, arsine (AsH.sub.3) is generally used as the arsenic-containing group V source material.
The use of dimethylhydrazine (DMHy) as the nitrogen-containing group V source material in place of ammonia is proposed in Document 5 (Appl. Phys. Lett. 70 (21), May 26, 1997, pp.2861-2863). Since dimethylhydrazine has a decomposition efficiency higher than that of ammonia, nitrogen atoms can be effectively incorporated into the crystal when forming a nitride mixed crystal semiconductor. In addition, Document 5 also proposes the use of tertiary-butylarsine (TBAs) in place of arsine as the arsenic-containing group V source material. Since tertiarybutylarsine has a higher decomposition efficiency than arsine, the consumption of the arsenic-containing group V source material may be advantageously reduced.
Moreover, a method of growing the crystal of a quaternary GaInNAs-type compound semiconductor as another example of a group III-V compound semiconductor is disclosed in Document 3 (Japanese Patent Laying-Open No. 9-283857). According to Document 3, an organic nitrogen compound such as dimethylhydrazine is used as the nitrogen-containing group V source material, and arsine is used as the arsenic-containing group V source material.
Now, when arsine is used as the arsenic-containing group V source material, only 50% of the arsine decomposes at 600.degree. C. Therefore, in order to obtain a high quality compound semiconductor crystal, the crystal must be grown at a high temperature as disclosed in Document 6 (Journal of Crystal Growth 115 (1991), pp. 1-11) or the amount of arsine supplied should be increased.
If, however, the growth temperature becomes higher during the growth of a compound semiconductor crystal of GaNAs-type, GaInNAs-type or the like, the incorporation of N into the crystal is suppressed. Thus, in order to prevent N from desorbing from the crystal growing surface, the amount of the nitrogen-containing group V source material supplied must be increased.
In addition, when the amount of arsine is increased, there is a possibility of the crystalline characteristic being affected by the reaction in the gas phase between the nitrogen-containing group V source material and dimethylhydrazine or the like.
Further, the amount of source gas supplied increases in either case in which the growth temperature is raised or in which the amount of arsine supplied is increased, inducing the need to consider the problems of the exhaust gas processing and environmental pollution.
Moreover, the inventors are particularly interested in a quartenary GaInNAs-type compound semiconductor among the compound semiconductor crystals. The GaInNAs-type compound semiconductor crystal can be lattice matched to a GaAs substrate. Therefore, a long-wavelength light-emitting device can be produced by controlling the composition of each element of Ga, In, N, and As.
The growth of a quartenary GaInNAs-type compound semiconductor crystal, however, involves a problem not found in a ternary GaAsN-type compound semiconductor, as will be described below.
Specifically, the inventors have found, as a result of the numerous experiments performed with regard to the growth of the GaInNAs-type compound semiconductor crystal, that it becomes more difficult for N to be incorporated into the crystal as the composition ratio of In in the crystal becomes higher, as described in Document 7 (A. Moto et al., "Enhancement of Nitrogen Incorporation in GaInNAs Grown by MOVPE Using Tertiary-butylarsine and Dimethylhydrazine," 25th International Symposium on Compound Semiconductors, Oct. 13, 1998 (Nara)). A similar problem is perceived in relation to the ternary GaInN-type compound semiconductor crystal which has the potential as a material for a blue laser device. In this latter case, it is observed that In is less easily incorporated into the crystal since the only group V element forming the crystal is N. This problem, however, is all together a different kind of a problem from that of the N incorporation described above.
As reported in Document 7, in order to incorporate N into the crystal effectively even when the composition ratio of In is high in the growth of the quartenary GaInNAs-type compound semiconductor crystal, there is a need either to increase the supply ratio of the nitrogen-containing group V source material to the arsenic-containing group V source material or to lower the growth temperature of the crystal so as to suppress the desorption of N from the crystal growing surface.
The approach of increasing the supplied amount of the nitrogen-containing group V source material or decreasing the supplied amount of the arsenic-containing group V source material can be considered in order to increase the supply ratio of the nitrogen-containing group V source material. From the cost and the environmental protection viewpoint, decreasing the supplied amount of the arsenic-containing group V source material is preferred. As described above, however, arsine used as the arsenic-containing group V source material has a poor decomposition efficiency. Therefore, decrease in the amount of arsine supplied may possibly lead to degradation in the crystal characteristic.
On the other hand, lowering the growth temperature of the crystal similarly retards decomposition of arsine, leading to the degradation of the crystal characteristic.
The growth of this material has been effected by molecular beam epitaxy (MBE) using a gas source material or by the organo-metallic chemical vapor deposition (OMCVD) method. It has been found that the optical characteristic deteriorates when the concentration of nitrogen is increased.
One method of determining the quality of the optical characteristic involves measuring the luminescence characteristic (photoluminescence). Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y and GaN.sub.y As.sub.1-y are generally evaluated by first directing a laser beam generated by an argon laser and having a wavelength of 514 nm into the crystal and thereafter measuring the intensity of luminescence emitted from the crystal.
The presence of defects or impurities in the crystal obstructs luminescence, causing the weakening of the intensity. Thus, by measuring the intensity, the quality of the optical characteristic can be determined. Moreover, the spread around the wavelength of luminescence (generally referred to as the peak half-width) correlates with the quality of the crystalline characteristic. A narrow peak half-width indicates a favorable crystalline characteristic.
Table 1 shows the correlation (measured at room temperature), as observed by the inventors, between the nitrogen composition and the luminescence intensity of GaN.sub.x As.sub.1-x not containing In and grown at 530.degree. C. It is appreciated from Table 1 that the luminescence intensity rapidly weakens and the optical characteristic deteriorates as the concentration of nitrogen increases. When the nitrogen concentration is high, no luminescence is detected.
TABLE 1 ______________________________________ Correlation between nitrogen composition and luminescence intensity of GaNAs grown at 530.degree. C. (measured at room temperature) Nitrogen Luminescence Intensity Concentration (%) (Arbitrary Unit) ______________________________________ 0.09 19.2 0.15 11.2 0.34 3.28 0.51 0 2.73 0 ______________________________________
Table 2 shows the correlation (measured at room temperature) between the nitrogen composition y and the luminescence intensity of Ga.sub.0.965 In.sub.0.05 N.sub.y As.sub.1-y grown at 530.degree. C.
TABLE 2 ______________________________________ Correlation between nitrogen composition and luminescence intensity of GaInNAs grown at 530.degree. C. (measured at room temperature) Nitrogen Luminescence Intensity Concentration (%) (Arbitrary Unit) ______________________________________ 0 GaInAs 20.3 3.5 GaInNAs 0 ______________________________________ In = 0.1, N = 0.035
It is apparent from Table 2 that luminescence similar to that depicted above is detected in GaInNAs.
Table 3 shows the correlation (measured at room temperature) of the nitrogen composition and the specific resistance of GaNAs grown at 530.degree. C.
TABLE 3 ______________________________________ Correlation between nitrogen composition and specific resistance of GaNAs grown at 530.degree. C. (measured at room temperature) Nitrogen Concentration (%) Specific Resistance (.OMEGA.cm) ______________________________________ 0.09 0.52 0.15 1.83 0.34 68.8 0.51 (*) 2.73 (*) ______________________________________ (*) indicates that specific resistance is larger than 400 .OMEGA.cm, whic cannot be measured.
Table 4 shows the correlation (measured at room temperature) between the nitrogen composition and the specific resistance of GaInNAs grown at 530.degree. C.
TABLE 4 ______________________________________ Correlation between nitrogen composition and specific resistance of GaInNAs grown at 530.degree. C. (measured at room temperature) Nitrogen Specific Resistance Concentration (%) (.OMEGA.cm) ______________________________________ 0 GaInAs 8.88 3.5 GaInNAs (*) ______________________________________ In = 0.1, N = 0.035 (*) indicates that specific resistance is larger than 400 .OMEGA.cm, whic cannot be measured.
As seen from Tables 3 and 4, the specific resistance of GaN.sub.x As.sub.1-x and Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y increases as the concentration of nitrogen increases, leading to the deterioration of the electrical characteristic.
Thus, when a thin film crystal having poor optical and electrical characteristics is used as the light-emitting device described above, the device undergoes a significant deterioration in its performance characteristic and reliability. In the case in which such a thin film crystal is used as a light-receiving device, its light-receiving sensitivity becomes so low that weak light cannot be detected. In addition, in the light-emitting device, the intensity of the emitted light becomes weak. Further, particularly in a semiconductor laser, continuous lasing operation becomes impossible. In both the cases of the light-emitting or light-receiving device and the electrical device, the device may not electrically operate if a layer having a high resistivity exists in the multi-layer film.
As shown in the above Document 3, as described in the above-mentioned growth method, when growing the Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y thin film crystal, As atoms can easily be desorbed from the surface of the thin film and the substrate. Thus, the growth at a low temperature is required to prevent the desorption. At such a low temperature, the desorption of -arsenic can be prevented and at the same time, the nitrogen source material more readily adsorbs to the crystal surface, advantageously leading to a higher nitrogen concentration. On the other hand, impurities also readily adsorb to the surface, which leads to an increase in the impurity concentration within the crystal in proportion to the increase in the nitrogen concentration.
In particular, in the organo-metallic chemical vapor deposition (OMCVD) method, many of the organic source gases used contain gas molecules consisting of hydrogen atoms. As a result, there is a problem of hydrogen being incorporated into the thin film crystal. The possible ways in which the contamination of hydrogen occurs may include the incorporation of the source gas itself, of the intermediate product produced during the decomposition of the source gas, or of the hydrogen atoms which have decomposed from the source material into the thin film crystal. It is considered that this hydrogen acts as an impurity to deteriorate the optical and electrical characteristics. Although no reports have been made with regard to hydrogen degrading the above characteristics within Ga.sub.1-x In.sub.x N.sub.y As.sub.1-y crystal or GaN.sub.y As.sub.1-y crystal, there have been instances in which such degradation was observed with other group III-V compound crystal semiconductors.
One reported example shows that hydrogen bonds with the doping atoms in a GaAs crystal, neutralizing the positive or negative charge of the doping atoms and becoming electrically inactive (J. I. Pankove, N. M. Johnson: Hydrogen in Semiconductors, 1991, Academic Press, Inc.). Moreover, another example is reported in which the optical characteristic is degraded as a result of N atoms within a GaP crystal binding with hydrogen (Jorg Weber, et al.: Mat. Res. Soc. Symp. Proc. Vol. 104, pp. 325). The mechanism of hydrogen atoms causing the degradation of the optical characteristic is yet to be investigated.